(a) Field of the Invention
This invention relates to a method for manufacturing a semiconductor structure, useful, in particular, for optoelectronic components, in which a substrate of a single-crystal semiconductor compound is provided with at least one layer of a further semiconductor compound by epitaxial deposition.
(b) Description of the Prior Art
It is well known that the conductive properties of a light-emitting layer in optoelectronic components can be improved if the layer is incorporated between mixed crystals having a different index of refraction. With such heterostructures, the losses can be so limited that threshold current densities of about 10.sup.3 A/cm.sup.2 are sufficient. The thickness of the active zone should not substantially exceed 1 .mu.m and should preferably be substantially less than 1 .mu.m, particularly 0.2 .mu.m or less. In these components, a substrate of a semiconductor compound is therefore provided, by epitaxial deposition, with further layers of the same semiconductor compound or of another semiconductor compound, such as a mixed crystal (solid solution) with approximately the same lattice constant, which forms a heterojunction. Thus, to improve properties of semiconductor lasers, such as threshold current and resonator geometry, and to limit losses, thin layers with uniform layer height and a plane surface without crystal faults are required.
This requirement must also be met in the manufacture of other electronic coponents in which thin layers of compound semiconductors are produced by epitaxial deposition such as electronic components using the avalanche effect and avalanche transit time diodes. These diodes contain a rectifying junction which is operated in the cutoff direction and have an efficiency which is essentially determined by the breakthrough behavior of the pn junction and thereby, by the homogeneity of the breakthrough region. The requirements mentioned above must also be met in the manufacture of microwave field effect transistors in which an intermediate layer serving as a diffusion barrier is deposited on a substrate of a compound semiconductor, e.g., chromium-doped gallium arsenide, followed by an active, n-conduction epitaxial layer. The electrodes are then applied to the active layer.
It is therefore an object of this invention to provide a new and improved process for epitaxial deposition of at least one thin layer of uniform thickness without crystal faults on the surface of a compound semiconductor.
As is well known, small deviations from the uniform thickness of epitaxially deposited thin layers, especially from the liquid phase, can reduce the quality of the components produced. Epitaxial layers grown on a crystal surface having an orientation error .alpha..phi. of about 30 min, show the terrace structures of the grown layer. On the other hand, a flat and plane epitaxial layer is obtained if the orientation error of the substrate surface is less than 5 min (Journal of Crystal Growth, 27 (1974), pages 148 to 253). The mentioned requirements can therefore be met if the layers are grown on a perfectly plane surface of a substrate having single crystal surfaces without orientation errors. Such semiconductor surfaces, however, can be produced only with a very large effort.
It is further known that, in the case of lasers having a layer sequence of semiconductor compounds with double-hetero structure, the growth terraces caused by random orientation faults of the substrate surface increase light losses due to scattering and the effectiveness of the active volume of the layers is reduced due to the corresponding variations of the layer thickness, contributing to an increase of the threshold voltage (Journal of Applied Physics, vol. 47, no. 9, September 1976, pages 3992 to 4005).